Magmatic-hydrothermal Evolution And Mineralization Mechanism Of The Dahutang Tungsten Granite, Jiangxi Province, South China

The South China is a famous tungsten deposit region in the world. The deposits are closely related to the Mesozoic granitic magmatism. Most of previous studies focus on the rare metal deposits in Nanling W-Sn polymetallic mineralization region in Cathaysia Block. Due to the discoveries of huge W and Mo deposits in the middle part of Qinhang metallogenic belt in recent years, further studies on rare metal mineralization in the Yangtze Block are in urgent need. This contribution presents detail mineralogy, petrology and geochemistry analyses on the Dahutang granite in the Lower Yangzte area, with aims to illustrate the effects of magmatic and hydrothermal evolutions on W mineralization, and reveal the behavious of W migration and enrichment during the magmatic and hydrothermal processes. In addition, comparision between the Dahutang and Xihuashan tungsten granites will give an insight into main mechanism for different mineralizations between the Qinhang and Nanling metallogenic belts.The Dahutang tungsten granitic pluton is characterized by multi-phase intrusions consisting of biotite granite, muscovite granite and lepidolite granite. The rocks are all strongly peraluminous(A/CNK=1.25 ~ 1.42) and highly evolved with high Si O2, alkali, F and P2O5. They are enriched in Cs, W, Nb, Ta, P, Li and Sn but depleted in Ba, Sr, REE and Ti, and have variable REE concentrations with significant negative Eu anomaly on chondrite-normalized REE patterns. Both muscovite granite and lepidolite granite show notable lanthanide tetrad effect. From biotite granite to muscovite granite to lepidolite granite, Rb/Sr ratio increases gradually with decreasing Zr/Hf and Nb/Ta ratios, denoting the trend of magmatic evolution. Accordingly, the contents of Li, Rb, Cs, P, W, Sn, Nb and Ta increase, but K2O、Mg O、Fe2O3、Ti O2、REE concentrations and(La/Yb)N and Eu/Eu* values decrease.Both mica and apatite in the Dahutang tungsten granites show compositional variation in consisten with magmatic evolution. From biotite granite to muscovite granite to lepidolite granite, mica’s Li, F, Rb, Cs and Ta contents increase but Fe OT, Mg O, Ti O2 and REE contents decrease; for the zoned apatite, core’s Ca O, Fe OT and P2O5 contents decrease but Mn O contents increasing accordingly. During magma differentiation, highly charged ion of Mn5+ replaces P5+ into apatite. However, in the hydrothermal process, low valence Mn2+ replaces Ca2+ into apatite. This indicates that the oxygen fugacity changes from high to low value due to fluid reaction. Primary rutile reserved in the interior of biotite contains relatively high Zr, corresponding to higher temperatures(680~760℃) of granitic melt; secondary rutile around ilmenite or accompanied with metal sulfide along the rim of biotite has relatively low Zr, agreeing with lower temperature(<490℃) of fluid reaction. Therefore, fluid reaction can reduce not only oxygen fugacity but also temperature of the melt.The micas in muscovite granite and lepidolite granite show significant zoning texture. Zoned mica exhibits gradually increasing Si, Rb, F, Fe, Li and W but decreasing Al from core to mantle, consistent with magma evolution. However, Si, Rb, F, Fe, Li and W contents decrease but Al content increases in the rim of zoned mica. Bulk-rock lanthanide tetrad effect and the occurrence of fluorite and metal sulfide are also consistent with the effect of fluid activity. In addition, zoned mica in Dahutang granite show gradual compositional variation from mantle to rim, which is different from that in Xihuashan W-bearing granite. Li and W contents in rim of Dahutang zoned mica are remarkblely higher than those of Xihuashan zoned mica, indicating that the fluid in Dahutang tungsten deposit would be derived from the exsolution of magma melts.During magmatic evolution, the enrichment of F can reduces the proportion of bridging oxygen, which will elevate the solubility of W and result in gradual enrichment of W in the melts. In addition, fluid reaction can reduce the oxygen fugacity, decreasing the partition coefficient of W6+ between metallic minerals and melts and consequently promoting the enrichment of tungsten in hydrothermal fluid. Then the divalent ions such as Fe and Mn are easier to combine with [WO4]2- to form wolframite. Xihuashan tungsten deposit is a typical quartz-vein type wolframite deposit. But Dahutang tungsten deposit consists of predominant veinlet-disseminated scheelite and minor quartz-vein type wolframite in association with Cu deposit. Such a distinctive mineralization in Dahutang deposit may be related to its particular country rocks such as the Neoproterozoic biotite granodiorite. Due to alteration by ore-forming fluid, the country rocks might release abundant Ca2+ that can combine with [WO4]2- to form scheelite. In addition, surrounding Neoproterozoic stratum of Shuangqiaoshan Formation is likely an ore-forming material source of W and Cu.